1. Field of the Invention
The present invention relates to a point-to-multipoint optical communication system to enable one transmission equipment at the central office to interactively communicate with a plurality of transceiver equipment at the subscriber premises.
2. Technical Background
There have been many R & D activities to develop point-to-multipoint optical communication systems in which one communication equipment at the central office can interactively communicate with a plurality of subscriber's equipment which are connected through branched optical paths to the central office equipment. A known example of such a system is a passive double star (PDS) system.
In the PDS system, branching of optical path lines is carried out by using an optical star coupler. The optical star coupler is a passive device which optically connects one optical fiber at the central office with any number (an integer) of subscriber's optical fibers, and performs the functions of optically merging subscriber's optical signal (referred to as upstreaming signals, or U-signals hereinbelow) and outputting the merged optical signal towards the central office. The optical star coupler also performs the functions of separating the optical signals from the central office equipment into the plurality of subscriber's optical fibers, and broadcasting the separated signals (referred to as downstreaming signals, or D-signals) to individual subscriber's equipment.
The PDS subscriber system will be explained with reference to FIGS. 21 and 22.
(A) PDS System using unidirectional star coupler PA0 (B) PDS System using bidirectional star coupler
FIG. 21 is a schematic illustration of the basic configuration of the PDS subscriber system using a set of unidirectional star couplers. To simplify the presentation, there are shown only two subscriber's equipment, however, it is quite possible to increase this number to any number of subscriber's equipment.
In this system, the set of star couplers 210a, 210b is disposed at a branching point in the optical path line to optically connect the central office equipment 100 with two subscriber's equipment 300, 400. Here, the star coupler 210a is an optical merger for merging the upstreaming signals, and optically connects the optical signal receiver 112 of the optical signal transceiver 110 at the central office with the optical signal transmitter 311 of the transceiver section 310 of the subscriber's equipment 300 and with the optical signal transmitter 411 of the transceiver section 410 of the subscriber's equipment 400. The optical star coupler 210b is an optical signal splitter for branching the downstreaming signals, and optically connects the optical signal transmitter 111 of the transceiver 110 at the central office with the optical signal receiver 312 of the transceiver section 310 of the subscriber's equipment 300 and with the optical signal receiver 412 of the transceiver section 410 of the subscriber's equipment 400.
In the above PDS system, prior to starting communication operation, propagation delay times, between the central office equipment 100 and each of the subscriber's equipment 300, 400, are measured, and commands are issued to prepare both of the equipment 300, 400 for signal transmission timing and data capacity for signal reception/transmission so as to avoid collisions of U-signals and superimposition of U-signals with the D-signals. The subscriber's equipment 300, 400 output U-signals at the requested transmission timing, and the U-signals are passively multiplexed by the optical merger 210a, and at the signal reception point of central office equipment 100, the U-signals from the equipment 300, 400 are arranged in time sequence. In the meantime, the D-signals forwarded from the central office equipment 100 to the subscriber's equipment 300, 400 are time division multiplexed, branched at the optical star coupler (optical splitter) 210b and are forwarded to the subscriber's equipment 300, 400. The subscriber's equipment 300, 400 select those signals which are addressed to itself from the time division multiplexed D-signals arranged on a time sequence.
FIG. 22 is a schematic illustration of the interactive PDS system using a bidirectional star coupler. In this illustration, the number of subscriber's equipment shown is limited to two for simplicity. This type of PDS system enables communication with a single line network by multiplexing both U- and D-signals. In this case, the optical star coupler 210 functions as an optical merger/splitter device. The methods of multiplexing the U- and D-signals can be divided into wavelength division multiplexing (WDM) method which assigns different wavelengths to each U- and D-signals, and time compression multiplexing (TCM) method which multiplexes the U- and D-signals on a specific time space.
In the PDS systems shown in FIGS. 21 and 22, to decrease the probability of generating communication problems, such as line breakdown and frequent bit errors, a method is known to duplicate the optical communication system comprising signal path lines, transmitting and receiving sections. FIG. 23 illustrates a case of duplicating the PDS system based on the configuration shown in FIG. 21, and FIG. 24 illustrates a case of duplicating the PDS system based on the configuration shown in FIG. 22.
Since the systems shown in FIGS. 23 and 24 are based on the same approach to duplicity, and it will suffice to explain the duplicating system on the basis of the PDS system shown in FIG. 24. In these illustrations, the system of bidirectional multiplexing is illustrated with a case involving one central office equipment and two subscriber's equipment, for simplicity.
As shown in FIG. 24, the central office equipment 100 and the subscriber's equipment 300, 400 respectively utilize optical fibers 200, 201 and 202, for the equipment 300, and optical fibers 220, 221 and 222 for the equipment 400. In each path line of the subscriber's equipment 300, 400, there are provided optical couplers 210, 230, respectively, for branching the D-signals to the subscriber's equipment 300, 400, and for coupling the U-signals to the central office equipment 100. The optical couplers 210, 230 are bidirectional optical couplers. In the following presentations, the system components including signal transceivers 110, 310, 410, optical fibers 201, 202, optical couplers 210, and optical fiber 200 will be referred to as the 0th path, and the system components including signal transceivers 120, 320, 420, optical fibers 221, 222, optical coupler 230 and optical fiber 220 will be referred to as the 1st path.
The central office equipment 100 comprises: a dual line transceivers 110, 120 for communicating with the subscriber's equipment 300, 400; a switching member 130 for switching the D-signal inputted from an input port S1; a switching member 140 for switching the U-signal to be forwarded to an output port R1.
The subscriber's equipment 300 comprises: a dual line transceiver sections 310, 320 for communicating with the central office equipment 100; a switching member 330 for switching the U-signals inputted from an input port S3; a switching member 340 for switching the D-signal to be forwarded to an output port R3.
Similarly, the subscriber's equipment 400 comprises: a dual line transceiver sections 410, 420 for communicating with the central office equipment 100; a switching member 430 for switching the U-signal inputted from an input port S4 (signal input); a switching member 440 for switching the D-signal to be forwarded to a receiving port R4 (signal output).
In the central office equipment 100, the transceiver 110 is optically connected to the transceiver 310 of the subscriber's equipment 300, and to the transceiver 410 of the subscriber's equipment 400, via optical fibers 200, 201 and 202, respectively. An optical coupler 210 is provided in the optical fiber 200 at the branching point to the subscriber's equipment 300, 400 so as to split or merge the U- and D-signals.
Similarly, the transceiver 120 is optically connected to the transceiver 320 of the subscriber's equipment 300, and to the transceiver 420 of the subscriber's equipment 400, via optical fibers 220, 221 and 222, respectively. An optical coupler 230 is provided in the optical fiber 220 at the branching point to the subscriber's equipment 300, 400 so as to split or merge the U- and D-signals.
Normally, the switching members 130, 140 of the central office equipment 100; the switching members 330, 340 of the subscriber's equipment 300; and the switching members 430, 440 of the subscriber's equipment 400 are all joined in the 0th path (the side labelled "a" in the switching members in FIG. 24). By this arrangement, the transceiver 110 is optically connected to the corresponding transceivers 310, 410, thus enabling optical communications via optical fiber 200, 201 and 202. In this case, it is possible to cease operation of the transceivers 120, 320 and 420 in the unused 1st path. It is of course desirable, from the viewpoint of conservation of power, that the unnecessary operations should be made to cease.
If a problem develops in any one of the transceivers 110, 310 and 410, or in any one of optical fibers 200, 201, 202 and optical coupler 210, the switching members 130, 140, 330, 340, 430 and 440 are all switched to the 1st path (the side labelled "b" in the switching members in FIG. 24). When the system is so switched, the transceiver 120 of the central office equipment 100 is connected to correspond with the transceivers 320, 420, and communication is carried out via optical fibers 220, 221 and 222.
Next, the operation of the system of the above construction will be explained with reference to a communication diagram shown in FIG. 25. This diagram relates to a case of the time division multiplexing method for multiplexing the subscriber signals, as well as for bidirectional multiplexing of U- and D-signals in accordance with the duplicate PDS system shown in FIG. 24.
In FIG. 24, the switching members 130, 140, 330, 340, 430 and 440 are all set on the "a" side in FIG. 24, signifying that the active system is the 0th path.
As shown FIG. 25, during the first half signal sending (broadcasting) period T.sub.S of a sending/receiving repetition period T, a D-signal D1 to the subscriber's equipment 300 and a D-signal D2 to the subscriber's equipment 400 are forwarded from the central office transceiver 110 and received by the subscriber's equipment 300, 400 in the following sequence.
Step 1
Signals D1, D2 are inputted into the input port S1 of the central office equipment 100 shown in FIG. 24, and are supplied to the transceiver 110 via switching member 130. PA1 Signals D1, D2 are converted to optical signals in the transmitter 111 of the transceiver 110, and are outputted to optical fiber 200 from the I/O port C110 via the optical coupler 113. PA1 Signals D1, D2 are branched by the optical coupler 210, and are supplied to the I/O port C310 of the subscriber's equipment 300, and to the I/O port C410 of the subscriber's equipment 400 via the respective optical fibers 201, 202. PA1 Signals D1, D2 are received by the signal transceiver 310 of the subscriber's equipment 300, and are branched by the optical coupler 313, are converted into electrical signals in the receiver 312, and signal D1 addressed to the subscriber's equipment 300 is discriminated, and is outputted to the I/O port R3 via the switching member 340. PA1 Similarly, in the transceiver 410 of the subscriber's equipment 400, signals D1, D2 are branched by the optical coupler 413, converted into electrical signals in the receiver 412, and signal D2 addressed to itself is discriminated, and is outputted to the I/O port R4. PA1 Signal U1 is inputted into the input port S3 of the subscriber's equipment 300 shown in FIG. 24, and is supplied to the transceiver 310 via the switching member 330. In the transceiver 310, signal U1 is converted into optical signal by the transmitter 311, and is outputted from the I/O port C310 to optical fiber 201 via the optical coupler 313. PA1 Similarly, signal U2 is inputted into the input port S4 of the subscriber's equipment 400, and is supplied to the transceiver 410 via the switching member 430. In the transceiver 410, signal U2 is converted into optical signal by the transmitter 411, and is outputted from the I/O port C410 to optical fiber 202 via the optical coupler 413. PA1 Signals U1, U2 forwarded to the respective optical fibers 201, 202 are coupled in the optical coupler 210, and are supplied to the I/O port C110 of the central office equipment 100 via optical fiber 200. PA1 Signals received in the transceiver 110 of the central office equipment 100 are branched at the optical coupler 113, photo-electric converted by the receiver 112, and are outputted to I/O port R1 via switching member 140.
Step 2
Step 3
Step 4
Next, during the latter half signal receiving period T.sub.R of the signal sending/receiving repetition period T, a U-signal U1 from the subscriber's equipment 300, and a U-signal U2 from the subscriber's equipment 400 are forwarded to the central office equipment 100 in the following sequence.
Step 5
In this case, to prevent the signals U1 and U2 from superimpose upon each other, they are outputted at predetermined time positions, i.e. time division multiplexed.
Step 6
Step 7
The above seven steps are repeated for each signal sending/receiving repetition period T.
For simplicity, such components as synchronizing frame for synchronizing signal sending/receiving, control channels for transmission timing assignment and switching operations in the subscriber's equipment and propagation delay time in the optical fibers are omitted from FIG. 25.
If there is any fear of developing problem in any one of the components such as the transceivers 110, 310, 410, optical fibers 200, 201, 202 and optical coupler 210, and it is deemed necessary to prevent degradation in the communication quality or shut down of a line, and to continue to provide high quality service, the system in use is changed from 0th to the 1st path.
In practice, if repairs are necessary to the optical fibers 200 while communication is being carried out using the 0th path, it will be necessary to switch the system from the 0th to the 1st path to maintain the continuity of reliable service. In such a case, all the switching members 130, 140, 330, 340, 430 and 440 are all switched to the "b" side shown in this figure, thus making the 1st path active.
FIG. 26 is a communication diagram in the case presented above. The communication operation is carried out as before the switching, by exchanging U- and D-signals between the transceiver 120 at the central office equipment 100 and the transceivers 320 and 420 on the subscriber's equipment 300, 400 via the optical fibers 220, 221 and 222.
After the switching operation, repairs can be made to any of the transceivers 110, 310, 410, optical fibers 200, 201, 202 and the optical coupler 210. The result is shortening of the down time, and improvement in reliability of service.
In the dual PDS system shown in FIG. 24, the two systems, the 0th path (comprising the transceiver 110 at the central office equipment 100, the transceiver 310 at the subscriber's equipment 300, the transceiver 410 at the subscriber's equipment 400, optical fibers 200, 201, 202 and the optical coupler 210) and the 1st path (comprising the transceiver 120 at the central office equipment 100, the transceiver 320 at the subscriber's equipment 300, the transceiver 420 at subscriber's equipment 400, optical fibers 220, 221, 222 and the optical coupler 230) are switched all at once by means of the switching members 130, 140 at the central office equipment 100, the switching members 330, 340 at the subscriber's equipment 300, the switching members 430, 440 at the subscriber's equipment 400.
After switching the systems over from one to the other, it is possible to save power by turning off the transceivers 310, 410 thereby shutting down the 0th path.
In such a dual PDS system, there are many components for constituting one optical path, and there is low probability that the switching operation will bring normal operation, because there is no assurance that all the components in the subscriber's equipment are operating normally. In the case of an example shown in FIG. 24, only two subscriber's equipment were shown for simplicity, but in actuality, there are many subscriber's equipment, and this probability drops even further. In other words, when there are many subscriber's equipment, even if the operating system is switched from one path to the other path, it is likely that the problem is still present somewhere in one or more components in the other optical path.
The result is that there is even a possibility that the normally operating path becomes dysfunctional by switching the the optical path. The disadvantage of this type of system is therefore, particularly severe in the case of one central office equipment serving many subscriber's equipment, thus making it difficult to offer highly reliable service. Also, the system configuration is such that switching is carried out for all the branches when a problem develops in one path, with the problem that there were frequent system shutdowns, and that the transmission quality suffered because of the attendant momentary interruptions.
The dual PDS system shown in FIG. 24 is a redundancy type in which two sets of transceivers are provided. In practice, however, it is possible to utilize a system having no redundancy at the subscriber's equipment. For example, if the user values high reliability of service, a redundancy type of system is provided while if the user values economy, a system having no redundancy is provided.
FIG. 27 shows an example of adding a subscriber's equipment 500 which has no redundancy to the dual PDS system shown in FIG. 24. As shown in this figure, the subscriber's equipment 500 has one transceiver 520, and the input terminal of the transmitter 521 of the transceiver 520 is connected to the input port S5, and the output terminal of the receiver 522 of the transceiver 520 is connected to the output port R5. Also, the transmitter 521 and the receiver 522 are connected to the optical coupler 230 via the optical coupler 523 and optical fiber 205. In other words, the transceiver 520 is a part of the 1st path.
In such a network configuration, if the 1st path is active, the subscriber's equipment 500 is able to communicate with the central office equipment 100, but if a problem develops in the 1st path, only the 0th path becomes active, and the subscriber's equipment 500 having no redundancy will become dysfunctional. That is, this type of system suffers from a disadvantage that a subscriber having no redundancy is unable to receive any communication service when the system is switched over from the 1st to the 0th path.
If the problem lies in its own equipment (for example, transceiver 520, optical fiber 205 etc.), an interruption in service may be accepted as a natural result of having placed importance on economy rather than reliability. However, the fundamental problem in the design of this type of system is that an interruption can brought about by problems existing in an equipment other than his own.
It is of course possible to mandate the use of redundancy in all subscriber's equipment, however, such a system will be costly, and the user's share of the cost becomes also high. Those subscribers who decided to opt for non-redundancy did so because of the economical attraction of the system, and if the cost of subscription becomes as high as that for a system having redundancy, it can be expected that the number of such existing and potential users will be greatly diminished. The end result is that the basic meaning of having options will be nullified for the subscribers.
As summarized above, reliable service is difficult to achieve using the conventional dual PDS system presented above, and those subscribers having no redundancy was subjected to interruptions in communication service even when the problem is not his own making.